Reactive polyurethane adhesives and sealants based on prepolymers with free isocyanate groups are distinguished by a very high performance profile. It has therefore been increasingly possible to open up new uses for these adhesives/sealants in recent years. Compositions for such adhesives and/or sealants are already known from a large number of patent applications and other publications. These also include, in particular, reactive, one-component, moisture-curing polyurethane hot melt adhesives.
These are solid at room temperature and are applied as an adhesive in the form of their melts; the polymeric constituents of polyurethane hot melt adhesives contain urethane groups and reactive isocyanate groups. By the cooling of these melts after application and joining of the substrate parts to be bonded, rapid physical setting of the hot melt adhesive initially takes place due to solidification thereof. This is followed by a chemical reaction of the isocyanate groups still present with moisture from the atmosphere to give a crosslinked, non-fusible adhesive. Reactive hot melt adhesives based on polyurethane prepolymers with terminal isocyanate groups are described e.g. by H. F. Huber and H. Müller in “Shaping Reactive Hotmelts Using LMW Copolyesters”, Adhesives Age, November 1987, page 32 to 35.
Laminating adhesives either can be built up in a similar manner to reactive hot melt adhesives, or they are applied as one-component systems from a solution in organic solvents. Another embodiment consists of two-component solvent-based or solvent-free systems in which the polymeric constituents of one component contains urethane groups and reactive isocyanate groups and the second component contains polymers or oligomers with hydroxyl groups, amino groups, epoxy groups and/or carboxyl groups. In these two-component systems the component containing isocyanate groups and the second component are mixed immediately before application, usually with the aid of a mixing and metering system.
In addition to many advantages, these polyurethane compositions also have some system-related disadvantages. One of the most serious disadvantages is the residual content of monomeric isocyanates, in particular in the case of the more volatile diisocyanates. Adhesive/sealants and, in particular, the hot melt adhesives are processed at elevated temperature. The hot melt adhesives, for example, are processed at between 100° C. and 200° C., and laminating adhesives are processed at between room temperature and 150° C. At room temperature, volatile isocyanates, such as TDI or IPDI, already have a vapour pressure which is not to be ignored. This noticeable vapour pressure is particularly serious in the case of application by spraying in particular, since significant amounts of isocyanate vapours can occur above the application object as a result, these being toxic because of their irritant and sensitizing effect. Protective measures must therefore be taken to prevent harm to the health of persons entrusted with the processing. These measures, such as e.g. the obligation to monitor observance of the maximum workplace concentration, are expensive. In particular, measures to suck off the vapours at the place where they are formed and discharged are very cost-intensive and furthermore impede some application processes, such as, in particular, application of reactive polyurethane adhesives/sealants by spraying.
For the fields of use mentioned, the development of reactive polyurethane compositions with a drastically reduced content of monomeric diisocyanates is therefore highly desirable, since in some cases their use is only made possible by the latter in many applications where the use was not hitherto possible because of the industrial hygiene problems explained above.
According to Schulz-Flory statistics, in the reaction of diisocyanates with isocyanate groups of about the same reactivity with compounds containing hydroxyl groups, the residual content of monomeric diisocyanate in the reaction product depends on the NCO/OH ratio of the reactants in the prepolymer synthesis. At an NCO/OH ratio of 2, such as is often necessary for the prepolymer composition, about 25% of the monomeric diisocyanates employed remains as monomer in the prepolymer. If e.g. 10 wt. % diphenylmethane diisocyanate (MDI) is employed in a prepolymer synthesis at an NCO/OH ratio of 2, about 2 wt. % of monomeric MDI is found in the prepolymer, in agreement with the abovementioned statistical estimation of the order of size. At 150° C. the pure MDI already has a vapour pressure of 0.8 mbar, although in compositions this vapour pressure is lower in accordance with Raoult's law, but it is still above the range acceptable in terms of industrial hygiene. Under the application conditions described above, in particular in the case of application as a hot melt adhesive in a thin layer over a large area, considerable amounts of the residual monomer thus enter the air space above the area and must be removed by sucking off. A significant lowering in the monomer content by a power of ten by reducing the NCO/OH ratio as a rule cannot be implemented in practice because the average molecular weight would then increase exponentially and the resulting polyurethane compositions would become extremely highly viscous and would no longer be processable. In practice, other routes are therefore also taken in the prepolymer synthesis. Thus, for example, the synthesis is carried out with a sufficiently high NCO/OH ratio and after the prepolymerization the monomeric diisocyanate is removed in a second step, which can be effected, for example, by distilling off the unreacted monomeric diisocyanate in vacuo or by subsequent chemical bonding of the monomeric diisocyanate. EP-A-316738 thus describes a process for the preparation of polyisocyanates containing urethane groups with a starting diisocyanate of not more than 0.4 wt. % which is free from urethane groups by reaction of aromatic diisocyanates with polyhydric alcohols and subsequent removal of the unreacted, excess starting diisocyanate, the removal of the excess starting diisocyanate by distillation being carried out in the presence of an aliphatic polyisocyanate containing isocyanate groups.
According to the disclosure of DE 10013186, removal of monomeric isocyanates by distillation can be avoided by reacting, in a first stage, an addition product of compounds with two isocyanate-reactive groups with a diisocyanate I, either the diisocyanate or the isocyanate-reactive compound having reactive groups of different reactivity. Equimolar ratios of diisocyanate and isocyanate-reactive compound are established here, so that the resulting addition product contains a group which is reactive with isocyanate and a free NCO group. An intermolecular addition reaction of this reaction product is said to follow, where appropriate, to give a polyaddition product which in turn contains an isocyanate-reactive group and an isocyanate group. This is followed by reaction with a further diisocyanate II, which differs from the abovementioned diisocyanate I.
EP-A-0393903 describes a process for the preparation of prepolymers, in which monomeric diisocyanate is reacted with a polyol in a first step. A catalyst is then added in a sufficient amount, so that a considerable portion of the residual isocyanate functionality is converted into allophanate functionality. When the theoretical NCO content has been reached, the reaction is stopped by rapid cooling and addition of salicylic acid.
WO-95/06124 describes polyurethane compositions with a low content of monomeric diisocyanates, which are prepared by reaction of polyols with trifunctional isocyanates and optionally addition of monofunctional chain terminators. A disadvantage of this process is the low availability of trifunctional, low molecular weight isocyanates, and in particular the trifunctional homologues of diphenylmethane diisocyanate are not commercially obtainable in a pure form.
According to the teaching of WO 01/40342, polyurethane compositions with a low content of monomeric diisocyanates can be prepared in a two-stage process, wherein a diol component with a molecular weight of less than 2,000 and a monomeric diisocyanate with a molecular weight of less than 500 are reacted in a first step. The unreacted monomeric diisocyanate is removed from this reaction product, and the high molecular weight low-monomer diisocyanate formed in this way is reacted with a polyol in a second step, so that a reactive prepolymer with isocyanate end groups is formed. According to this specification, such polyurethane compositions are suitable for use as binders for reactive one- or two-component adhesives/sealants, which can optionally be solvent-based, and these compositions are furthermore said to be suitable for the preparation of reactive hot melt adhesives if the polyols are chosen accordingly.
The still unpublished DE 101 32571.1 proposes, for the preparation of reactive polyurethanes with a low content of monomeric isocyanates, reaction of at least one monomeric, asymmetric diisocyanate with a molecular weight of 160 g/ml to 500 g/mol with at least one diol with a molecular weight of 50 g/mol to 2,000 g/mol. Due to the high selectivity of the reaction, no additional working up and purification steps to remove the excess monomer are necessary. These reaction products can be reacted directly in a second stage with higher molecular weight polyols to give the end product.
Although the products which can be prepared according to the disclosure of the last two specifications mentioned have very good processing properties and a low content of monomeric diisocyanates, it is desirable to further simplify the preparation processes for low-monomer reactive polyurethane compositions. This includes a simple reaction procedure, the lowest possible viscosity of the reaction product and a good melt stability of the reactive polyurethane composition.
EP 693511 A1 describes reactive hot melt systems containing isocyanate groups. These compositions are reaction products of hydroxypolyols with ester and/or ether groupings, a hydroxyl number of 15 to 150 and an average functionality of 1.95 to 2.2 with diphenylmethane diisocyanates in a ratio of isocyanate groups in the diphenylmethane diisocyanates to hydroxyl groups in the polyols of 1.4:1 to 2.5:1, the diphenylmethane diisocyanate (MDI) having a content of at least 70 wt. % of 2,4′-diphenylmethane diisocyanate. According to the teaching of this specification, these reaction products have a low initial viscosity and an increased heat stability (melt stability), so that they are said to suitable as solid adhesives for the most diverse fields of use. No information is given in the specification about the residual content of monomeric diisocyanates of these products.
In spite of the abovementioned prior art, there is thus still a need for improved polyurethane compositions with a low content of monomeric diisocyanates which are suitable for use as adhesives/sealants, in particular for reactive hot melt adhesives. In particular, the raw materials employed here should be readily and inexpensively accessible and easy to react, and the adhesion properties should be at least equivalent to those of conventional hot melt adhesives. The inventors therefore had the object of providing polyurethane compositions which can solve the last-mentioned problems.